The following invention relates to a method and apparatus for digital time domain reflectometry and in particular to a method and apparatus for obtaining an impedance signature of a cable under test.
The use of time domain reflectometers is a well-known method of establishing the location of faults and discontinuities in electrical cable. Such time domain reflectometers typically comprise a current pulse generator connected to the cable under test and an oscilloscope also connected to the cable to observe the shape of a voltage pulse caused by reflection at an impedance mismatch or discontinuity. Examples of these systems are shown in the Frye U.S. Pat. No. 3,434,049, Wrench, et al. U.S. Pat. No. 4,109,117, McFairen U.S. Pat. No. 3,727,128, Gayle U.S. Pat. No. 4,165,482, Lambertson U.S. Pat. No. 4,104,582, Cormack U.S. Pat. No. 3,617,880, Cronson U.S. Pat. No. 3,903,477 and Cronson U.S. Pat. No. 3,812,423. There are also commercially available time domain reflectometers such as those made by Tektronix, Inc. of Beaverton, Oreg. which are sold under Model Nos. 1502 and 1503.
The problem with all of the aforementioned devices is that the reflected voltage pulse is observed only as a trace on an oscilloscope, and a judgment must be made regarding the severity of the potential problem that the reflection represents. This requires a skilled operator who can interpret the data. For example, there may be many such reflections which represent minor cable impedance mismatches which may not be serious enough to warrant repair. Thus, such devices require skilled operators to observe and interpret traces on a CRT in order to determine the magnitude or character of the potential problems that the traces represent.
Unless a cable is perfect it will have minor impedance mismatches at various points along its length. It would be helpful in such cases to obtain an impedance "signature" for the cable that would tell a user where the impedance mismatches were located, and quantify them as to type and severity. Largely for the reasons stated above, conventional time domain reflectometers employing an oscilloscope are incapable of obtaining this data.
Yet a further problem is that most time domain reflectometers cannot be used in a network which includes a plurality of devices operatively in communication with one another. For example, most time domain reflectometers could not be used in a network which included a central computer and a number of satellite terminals connected over a network such as an IEEE 802.3 standard network. This severely limits the use of the time domain reflectometers described above to those situations in which the network can be disconnected and a test signal inserted. This is both time-consuming and cumbersome.